FIGS. 20 to 23 schematically illustrate an axial flow fan in the related art.
FIG. 20 is a perspective view of a boss-equipped axial flow fan in the related art.
FIG. 21 is a front view of the boss-equipped axial flow fan in the related art, as viewed from upstream in a fluid flowing direction.
FIG. 22 is a front view of the boss-equipped axial flow fan in the related art, as viewed from downstream in the fluid flowing direction.
FIG. 23 is a side view of the boss-equipped axial flow fan in the related art, as viewed from a lateral side relative to a rotation axis.
As shown in FIGS. 20 to 23, the axial flow fan in the related art includes a plurality of blades 1 along the peripheral surface of a cylindrical boss. When a rotational force is applied to the boss, the blades 1 rotate in a rotational direction 11 to convey a fluid in a fluid flowing direction 10. Such a configuration is also disclosed in, for example, Patent Literature 1. In the axial flow fan, the blades 1 rotate to cause the fluid existing between the blades to collide against the blade surfaces. The surfaces against which the fluid collides increase in pressure and press and move the fluid in the direction of a rotation axis serving as a central axis when the blades 1 rotate.
In terms of the shape of an axial flow fan, a so-called boss-less fan not having a cylindrical boss is also known (see Patent Literature 2). In a boss-less fan, leading edges and trailing edges of neighboring blades among a plurality of blades 1 are connected by a continuous surface without the intervention of a boss, and the boss-less fan is provided with a small-diameter cylindrical portion at the center thereof for securing a drive shaft of a motor thereto. Thus, the minimum radius of the continuous surface between the blades centered on a rotation axis is larger than the radius of the cylindrical portion for securing the drive shaft thereto.